Power transmission distribution cable includes an electrical conductor that is commonly surrounded by a semiconducting polymer, by insulation, and finally by an outer protective jacket. The high potential usually carried by the conductor, and the environment in which the cable is located contribute to deterioration of the cable insulation. If the transmission cable is not replaced before the insulation deteriorates significantly, catastrophic failure and damage can result. Similarly, even transmission cable conducting a lower potential within a radiation environment can cause catastrophic failure if it is not replaced before its insulation deteriorates significantly.
Techniques are known in the art for diagnosing insulation failure after the fact. After failure, the insulation's electrical breakdown characteristics are analyzed to try to learn the mode of failure. Infrared spectroscopic studies of the insulation may be conducted, in addition to analysis of the insulation deterioration (known as "water treeing"). At best, the results of such studies may be useful in learning why the insulation failed.
However there is no known technique whereby cable insulation may be examined, preferably before catastrophic damage, and the remaining insulation lifetime predicted. For example, if the insulation fails in a segment of installed cable, what should be done? If the failure was due to aging, then the entire cable installation should be replaced (since all of the insulation is equally old and will fail soon). On the other hand, if the failure was localized, it would suffice to splice in a new section of cable to replace the failed cable segment. Unfortunately, the prior art does not provide a diagnostic method by which the remaining insulation lifetime may be quantized.
There is a need for a method of predicting the remaining lifetime of transmission cable based upon examination of its insulation. Preferably the method should enable such analysis to be made even when the transmission cable is buried underground, or is within a hostile environment such as a nuclear reactor. The present invention provides such a method. Further, the present invention provides a transmission cable facilitating such analysis, as well as a method for fabricating such cable.